Improve error handling

[simolus3]

In powersync-ja/powersync.dart#453 (comment), we've gotten a report where the core extension reports an error like this (emphasis is the part from the extension):

SqliteException(1): while selecting from statement, powersync_control: internal SQLite call returned ERROR, SQL logic error (code 1)
Causing statement: SELECT CAST(powersync_control(?, ?) AS TEXT), parameters: line_binary, blob (55 bytes)

That doesn't help much to understand the problem: Some statement ran by powersync_control has failed, but which one?

This PR removes the From<ResultCode> implementation for PowerSyncError, forcing us to provide more specific details when returning errors. Essentially, this statically prevents forgetting to call into_db_result. But even into_db_result, which adds more context into errors, wouldn't include the failed statement. So, this adds our own database (Database, replacing *mut sqlite3) and statement (Statement, replacing ManagedStmt) wrappers which, instead of returning a ResultCode error, will automatically return a PowerSyncError with appropriate context and the causing SQL statement.

These wrappers also simplify some things:

1. Instead of returning a ResultCode in the Ok case, step() now returns a Result<bool> indicating whether a row is available.
2. Previously, calling db.exec with a &str would allocate a CString to pass to SQLite. This replaces the method with something taking a &CStr directly (avoiding a copy for literals).
3. Allow all PowerSyncErrors to be wrapped with additional context, this was previously only possible for ResultCode-backed errors.

AI use: The new helpers are manually written, migrating the rest of the crate was done mechanically with string replacements and AI.
I also ran this through a Code Review with Claude Code to confirm existing behavior is unchanged.

[stevensJourney]

This looks like a nice improvement. Changes look good to me.